Password Protecting Your Work
All Macs come with extensive built in mechanisms for protecting your work, there is no need for individual programs to do this. The best and easiest way to protect your work is to enable FileVault for your account, or your entire computer (introduced with 10.7).
- Protecting the computer will set the entire hard drive to be scrambled except for a few key bootstrap pieces at the very top of it (so that it can be turned on). To even boot the computer it needs to be decrypted. This is by the far the safest setting, but also the most dangerous if you forget the password and global reset code.
- Protecting your account is the next safest, and of course this implies that you do not set your computer up to automatically boot up straight to your desktop without typing in any credentials. Again, the risk here is that if you forget your password or reset code then everything in your user folder is forever lost.
- Finally there is protected disk images, which can be created like little "vaults" that you can store stuff in. Any projects in Scrivener, your tax documents---anything that shouldn't be on your computer as readable bits should be in an encrypted vault (if one of the above protections are not used, that is). These can be easily created using Apple's Disk Utility software and its documentation. If you find it all a bit too complicated, there are programs you can buy which streamline this process for you.
Of course, simply putting a password on your user account and setting your Mac to not automatically log you in is protection enough for most purposes. If all you want to do is make sure an inquisitive child does not log into your computer and destroy your novel, that might be all you need to do.
Scrivener projects (.scriv files) are packages of files. On Windows, a project on disk just looks like any other folder. On macOS, while a project looks like a regular file, you can Ctrl-click on it in the Finder and select "Show Package Contents" to reveal the files inside it. Inside you will find a number of folders and RTF files that you could open in any text editor or Word Processor (although you should not edit files in this way, as it could cause problems with the project).
This means that if Scrivener allowed you to set a password to open a particular project, it could provide only a very superficial level (or, at worst, a false sense) of security, since the files could still be accessed via the Finder.